A typical operational amplifier is shown in FIG. 1. As will be readily appreciated by those skilled in the art, the relationship between the output signal and the input signal is commonly expressed as: ##EQU2## Note that the positive input to the amplifier is tied to analog ground ("AGND"), which is typically half way between positive and negative supply rails. The amplification is thus dictated by the resistance ratio of R.sub.3 to R.sub.1.
For high dynamic range applications, differential circuits such as the one symbolically shown in FIG. 2 are desired. The need for differential circuits is simple: For an input voltage with peak-to-peak voltage of 2 v, the outputs, INP and INM, will produce a differential output range from +2 v to -2 v, which doubles the effective dynamic range of the input signal without having to increase the supply voltage of the amplifier. This higher dynamic range of an amplifier is quite desirable in low-power audio or speech processing circuits.
FIG. 3 shows a circuit diagram of a typical differential amplifier with differential inputs, INM and INP, and differential outputs, OUTP and OUTM. The gain of this circuit is commonly expressed by: ##EQU3## Note that the common-mode input voltage, V.sub.CM (=(V.sub.A +V.sub.B)/2)) is fixed, and independent of the differential input in the case of fully differential amplifiers, as commonly known to those skilled in the art.
A conventional single-ended to differential amplifier is shown in FIG. 4, which originates from the combination of FIGS. 2 and 3. Note that the input to the amplifier at the positive terminal is now connected to AGND. The gain of this amplifier of FIG. 4 is still: ##EQU4## since IN is relative to AGND.
However, the common-mode input voltage V.sub.CM, i.e. (V.sub.A +V.sub.B)/2, is now changing with the input voltage, IN, in the following way: ##EQU5## Thus, if IN decreases, V.sub.CM also decreases due to its relation to OUTP and vice versa. To put it in practical terms, the common-mode input voltage of the differential amplifier of FIG. 4 will change according to the input signal, in contrast to the differential input-independence of the fully differential amplifier of FIG. 3.
This dependence requires a large common-mode input range for the single-ended to differential amplifier, especially in the cases of applications with a low supply voltage such as 2.7 v. A large common-mode input voltage forces the transistors used for the tail current of the differential pair to operate in the linear region, instead of the strong inversion region. Therefore, conventional single-ended to differential amplifiers require complementary input stages to deal with the large common-mode input signals to the amplifier, resulting in extra power and more components.
For high dynamic range applications, it is desirable to have a single-ended to differential amplifier with low common-mode input requirement without incurring extra power consumption or increasing component complexity.